Neural stem cells are self-renewing multipotential progenitor cells, whose daughter cells can differentiate into both neurons and glia (Gage, F., Science 287:1433-1438 (2000); McKay, R. D., Cell 58:815-21 (1989); McKay, R., Science 276:66-71 (1997); Weiss et al., Trends Neurosci 19:387-93 (1996); Ourednik et al., Clin Genet 56:267-78 (1999)). An important feature of neural stem cells is their ability to replicate themselves by symmetric division and clonal expansion, but no humoral agents have yet been defined that specifically support these self-renewing divisions. Mitogens, including EGF and FGF, support the continued proliferation and expansion of neural stem cells, though cofactors appear to be required for low density culture (Taupin et al., Neuron 28:385-397 (2000)). Neural stem cells have proven particularly amenable to growth as free-floating clusters in suspension culture, designated neurospheres by Weiss and colleagues (Morshead et al., Neuron 13:1071-82 (1994); Reynolds et al., J Neurosci 12:4565-74 (1992); Reynolds et al., Science 255:1707-10 (1992); Laywell et al., Proc Natl Acad Sci USA 97:13883-8 (2000)), following their observation that the attachment of neural stem cells may result in the loss of their capacity for self-renewal. Even in the presence of mitogens, neural stem cells typically differentiate into committed progenitors and their neuronal and glial progeny when raised in monolayer cultures. The need for being in close contact with one another may indicate that a short-range autocrine/paracrine signaling mechanism is required for continued expansion of neural stem cells. The close-acting wnt/frizzled pathway has been implicated in the suppression of neural phenotypes from ES cells, but not in neural stem cell self-renewal (Aubert et al., Nat Biotechnol 20:1240-5 (2002)). Glycosylated cystatin C(CCg) has been proposed to be another locally acting agent, which appears to act as a co-factor to potentiate FGF2-activated progenitor cell division, permitting FGF-dependent expansion to operate down to very low cell densities (Taupin et al., Neuron 28:385-397 (2000)). However, although CCg appears to be a promising autocrine/paracrine agent promoting progenitor cell expansion, it acts as a co-factor, and does not operate alone as a self-renewal factor.
The present invention is directed to overcoming these deficiencies in the art.